Bridge or Tunnel?

Should a motorway pass under or over a large waterway? For a narrow waterway there is no problem, the water is always bridged. Until 1960 only two alternatives existed, bridging over or tunneling under the waterway, but now a third choice is available, the immersed tube is made by lowering pipes of great length into a trench in the bed of the sea or river, and joining them under water.

The main considerations in the decision are now generally traffic capacity, gradient, obstruction of shipping, costs of construction and maintenance, speed of completion, possible later widening and so on.

The choice often falls on a bridge because it can carry more vehicles per hour and its capacity is more easily extended by widening or by adding a deck.

One of the largest bridges in the world, with twelve traffic lanes, six on each of its two decks, is the Verrazano-Narrows Bridge in the New York. For many years the United States Army engineers would not allow a bridge to be constructed at this site because its destruction in war time could block the harbor, and they insisted on building a tunnel. But after the first atomic bombs had been exploded, the U.S. Army saw that it was purposeless to continue to insist on a tunnel.

In the eightieth of the last century, also for military reasons, the British government would not allow further work on the tunnel under the English Channel to France although the preliminary work was by then so advanced that lengths of 1 km of pilot tunnel of some 2 m diameter had already been driven from each shore. These lengths were still in perfect condition when inspected eighty years later, because they had been driven through a chalk rock which is ideal for tunneling, being fairly watertight and just strong enough.

The ever increasing motor traffic needs an ever growing number of highways, which will have to cross important waterways, with also increasing shipping. In several cases tunnels will not only be the cheapest solution, but also the best with regard to weather conditions (no ice or snow, no wind or rain), maintenance, danger of collision with a ship, aesthetic reasons, etc.

Bridge/tunnel combinations form attractive and often obvious solution for crossings of great length.

It is easy to predict that in the next decades an ever increasing number of important and interesting tunnels — submerged or bored — will be built, and that the existing methods of building, sinking, etc. will be improved and perfected and new and astonishing techniques will be developed.

Prestressed concrete runways and concrete pavements

This prestressed concrete runway, the first in the world, was designed by Freyssinet and was constructed as an experiment. It is 14 000 ft. long and 200 ft. wide and is divided into large triangular slabs separated by special joints. The large triangular slabs are composed of smaller, square precast slabs, measuring 39 in. on edge. Each slab has a uniform thickness of 6,33 in. The large triangular slabs can move laterally only by sliding along the joints between slabs. These joints rest on concrete foundations and are formed by placing short vertical steel rollers between steel plates fastened to the edges of the triangular slabs. The joints are filled with asphalt, which permit the slabs to move relative to one another. The runway is prestressed transversely by 200 ft. long cables, composed of 30 wires each placed between the joints of the small precast slabs and anchored in border units, which are required to “pack” these slabs and distribute the cable forces. The prestressing is obtained automatically by the wedge action which accompanied the transverse prestressing.